Position encoders are used in a variety of applications to provide feedback identifying the position of a moveable member. Such moveable members may comprise a steering wheel for a vehicle, a rotor shaft in a brushless electric motor, or other structures in which the position and/or velocity of a rotating member relative to a stationary member needs to be tracked or identified. The encoder may typically be in the form of a magnetic encoder comprising a sense magnet assembly including a plurality of magnetized regions located in a circumferential alternating pattern. A Hall sensor may be provided adjacent to the sense magnet assembly for determining the rotational position of a rotating shaft or rotor of the moveable member.
In a particular construction of a known sense magnet assembly, as seen in FIG. 1, a sense magnet assembly 10 comprises a layer of magnet material 12, a ferrous back iron 14, and a non-magnetic hub 16. The inner diameter 18 of the hub 16 may comprise an opening that may be positioned onto the rotating shaft (not shown) of a moveable member, such as the rotor of an electric motor (not shown). Adhesive may be used to create a bond in the areas of contact between the sense magnet material 12, the back iron 14, and the hub 16. In a typical exemplary application, the sense magnet assembly 10 may be rigidly attached to a rotating component (not shown) that is fixed to a motor shaft for rotation with the motor shaft. The sense magnet assembly 10 may be attached to the rotating component by any conventional means, such as by means of an adhesive or threaded fasteners (not shown) passing through holes 20 in the sense magnet assembly 10 for engagement with the rotating component.
The magnet material 12 is magnetized to provide a circumferential pattern of alternating north and south poles that may be detected by a stationary sensor structure 22 comprising Hall sensors 24 located near the top face of the magnet material 12 in order to provide information about the speed and position of the rotating motor shaft. The magnet material 12 may be magnetized with any number of circumferential patterns existing at various diameters within the magnet material 12.
The magnet material 12 may comprise a relatively elastic material held in place on the back iron 14 by an adhesive. As used herein, the term ‘elastic’ may describe a material that is soft, flexible, or non-rigid, or a material that may become soft, flexible, or non-rigid when exposed to the environmental or operating conditions of the end product, i.e., an electric motor, such as high temperature operating conditions.
The adhesive used to hold the magnet material 12 in place may comprise a weak adhesive, where the term ‘weak’ is used herein to describe an adhesive with low tensile strength, an adhesive that does not bond well to one or all of the above-described sense magnet assembly components, or an adhesive with low tensile or bond strength when exposed to the environmental conditions of the end product, such as high temperature operating conditions. As a result of forming the described known sense magnet assembly 10 with, for example, an elastic magnet material 12 attached with a weak adhesive or no adhesive, the magnet material 12 may become detached from the adjacent structure in response to centrifugal forces applied by high rotational speeds of the motor shaft. As illustrated in FIGS. 2A and 2B, as the motor shaft rotates, the elastic magnet material 12 may buckle or bulge, as illustrated at 26 in FIG. 2A, or may lift away from the underlying back iron 14, as illustrated at 28 in FIG. 2B. Further, the magnet material 12 is typically formed as a single continuous disk of elastic material, such that the material closest to the axis of rotation will generally push outwardly on the material forming the radial outer portions of the magnet material 12, causing the radial outer portions of the magnet material 12 to be forced axially out of the motor assembly 10 and possibly strike another component of the sense magnet assembly 10, such as the sensor structure 22, or may even distort or deflect the hub 16. Contact between the magnet material 12 and another component of the motor assembly, such as the sensor structure 22, could cause catastrophic damage within the motor assembly, resulting in failure of the motor.